High purity tantalum metal and tantalum pentoxide have become increasingly important to the electronics industry in the preparation of advanced electronic materials used in the manufacture of devices such as surface acoustic wave filters, pyroelectric infrared sensors and optoelectronic devices. High purity tantalum pentoxide is also required for the preparation of tantalate X-ray phosphors for X-ray intensifier screens. The purity of tantalum metal and tantalum pentoxide used in the manufacture of such products should be very high, especially, with respect to certain transition metals such as niobium.
Prior to 1957, niobium was separated from tantalum by a fractional crystallization process known as the Marignac process which exploits the difference in solubility between K.sub.2 TaF.sub.7 and K.sub.2 NbOF.sub.5.H.sub.2 O. However, tantalum pentoxide obtained by this process contained large impurities of Nb (1000-3000 ppm), and other elements such as Si (up to 3000 ppm), Ti (up to 100 ppm), and Fe (up to 2000 ppm).
In late 1950's, modern solvent extraction and ion exchange processes supplanted the use of the Marignac process. Examples of liquid--liquid solvent extraction and ion exchange methods are disclosed in U.S. Pat. Nos. 3,117,833, 3,712,939, 4,673,554 and U.S. Pat. No. 4,446,115. In a solvent extraction process, ore concentrates containing at least 25 wt.% tantalum and niobium pentoxide are decomposed chemically in hydrofluoric acid media and the dissolved tantalum and niobium species are separated from the residue by filtration. The filtrate containing tantalum (as TaF.sub.7 .sup.2-) and niobium (as NbOF.sub.5.sup.2-) in an HF/H.sub.2 SO.sub.4 medium is brought into contact with an organic phase, usually methyl iso-butyl ketone (MIBK), which selectively adsorbs tantalum and niobium species leaving impurities such as titanium, iron, and silicon in the aqueous phase. Niobium is separated from tantalum by back extraction with sulfuric acid. Finally, tantalum (TaF.sub.7.sup.2-) is eluted from organic phase (MIBK) by an ammonium fluoride solution and converted into hydrated ammonium tantalum oxide by precipitation with ammonium hydroxide. The hydrated ammonium tantalum oxide is amorphous and contains a significant amount of ammonia, water and fluoride which are removed by calcination between 750-1300 .degree. C. which converts the amorphous material to a low temperature crystalline phase of tantalum pentoxide or .beta.-Ta.sub.2 O.sub.5.
The tantalum pentoxide prepared by this method normally contains impurities such as Al, Si, F, Cl, K, Na, Cr, Fe, Co, Ni, Cu, Ti, Zr, Mo, Nb, and W with total weight of impurities at about between 0.1 to 1%. Although Ta.sub.2 O.sub.5 (or K.sub.2 TaF.sub.7) made by solvent extraction method can be used for most applications, this material is not suitable for the preparation of electronic materials. Optical grade tantalum pentoxide which is lower in transition metals, and some other elemental impurities depending on the specific application, is normally required for the preparation of electronic materials such as surface acoustic wave filters, pyro-electric infrared sensors, and opto-electronic devices, and X-ray phosphors. For example, for X-ray phosphors, the total transition metal impurities by weight should not exceed 10 to 20 ppm.
Preparation of optical grade tantalum oxide requires either a sophisticated chlorination process or multiple extraction/back extraction cycles by solvent extraction. Because of this, the cost of optical grade tantalum oxide is remarkably high as compared to standard technical grade tantalum pentoxide. Moreover, it is almost impossible to completely remove niobium by chlorination or multiple extraction/back extraction cycles of solvent extraction.
U.S. Pat. No. 5,635,146 which is incorporated herein by reference describes an alternative method for the purification of tantalum pentoxide. However, extra steps are required to convert the impure tantalum pentoxide to a soluble potassium tantalate prior to dissolution.